A turbine section of a gas turbine typically has a plurality of rows of stationary vanes and rotary blades. The blades of one row are usually identical to each other and include an aerofoil portion, a platform portion, and a root portion. Some blade rows may additionally include a shroud portion preventing the hot gases escaping over the blade tip. In reference to an axis of rotation which may define an axial direction within the gas turbine engine, the root portion is the most radial inward section of the blade, i.e. directed to the axis of rotation. A radial direction may be defined as being perpendicular to the axis of rotation. The root portion is used to mount the blade in a mounting groove or slot provided in a rotor disc. Typically for each rotor blade a corresponding mounting groove is provided. The blades are particularly assembled by axially sliding each root portion into the corresponding groove.
It is known for turbine blades to be fitted to turbine discs by means of cooperating firtree profiles. Such fixing methods provide accurate location of the blade with respect to the disc. Firtree profiles are sufficiently strong to withstand the radially outward—centrifugal—forces imposed on the blade during rotation of the disc and its attached blades in operation of the turbine engine in which it is installed. In operation, flanks of the firtree profiles of the blades which face away—in a slanted manner—from the axis of rotation and which are in contact with opposite firtree profiles of the grooves, support the blades against radially outward movement, and can be regarded as loaded flanks. The oppositely facing flanks of the profiles can be regarded as unloaded flanks, since they do not support any significant radial forces in operation.
The conventional shape of a turbine blade firtree root is defined using straight lines and circular arcs only, when looked at in a sectional view of the blade root, the sectional view is defined by a plane perpendicular to the rotor axis of the turbine. Such a shape is optimised against a number of geometric and mechanical constraints.
The flanks of the profiles are interconnected by transition regions which are alternately convex surfaces, which are usually but not always arcuate and are referred to as fillets or necks, and concave surfaces, which are usually but not always arcuate and are known as corners or lobes or teeth or lugs. The fillets are typically regions of high stress concentration.
A root may be substantially mirror-symmetrical. The root comprise a pair of symmetrical uppermost necks or fillets which extends downwardly from a lower surface of a platform and form a recess in circumferential direction, a pair of uppermost lugs or lobes which extend downwardly from the uppermost necks and form a projection in circumferential direction. A plurality of symmetrical pairs of necks and lobes may follow downwardly in alternating order. The root portion will end via a pair of symmetrical lowermost necks followed by a pair of symmetrical lowermost lobes. Surfaces of the pair of lowermost lobes will converge and will be joined at a most downward location via an arcuate or flat surface, the root bottom.
Rotor blades in a turbine section are affected by hot working fluid in a main gas path. This may require cooling. Rotating part may be difficult to cool. In order to lengthen the life of the blade, the blade is often cooled by passing a cooling fluid through cooling ducts provided inside the blade aerofoil. To supply cooling fluid to the ducts or hollow interior of the blade, cooling fluid may for example be provided via passages within in the root of the blade.
A common means of supplying the cooling air to the rotor blades is via holes at a rim of the turbine disc which transmit the cooling air from a separate internal cavity and into passages provided at the base of the blade roots.
In more detail, in order to supply cooling fluid to an inlet 28 within a bottom of the blade root 1 of a rotor blade 2, as illustrated in FIG. 1A and in a cross sectional view in FIG. 1B, a duct—see reference sign 42 in FIG. 1C—may be provided in the turbine disc 5, which carries cooling fluid from outside the disc to the disc slot 40, from where it flows into the inlet 28. An example of a disc according to this arrangement is given in FIG. 1C. This example is taken from U.S. Pat. No. 4,344,738 to assignee United Technologies Corp. and shows the fir-tree shaped disc slot 40, the duct 42 in the disc, an outlet 44 at the radially outer end of the duct 42, and an inlet 46 in an end-face of the disc. In operation the cooling fluid—typically air taken from a compressor section of the turbine engine—enters the inlet 46, passes through the duct 42 and leaves at the outlet 44, where it finally enters a cooling passage in the corresponding rotor blade, which is inserted in the slot.
It is recognised that a high stress may exist within the disc at the outlet holes during operation. It is a goal to minimize the stress concentration in the area of the outlet, particularly induced by hoop stresses.
Patent application EP 1 892 375 A1 already provides a solution by eliminating the acute corner produced by the radial intersection of the cooling hole and the disc slot bottom by introducing a cut-out feature in the disc slot bottom.